The present invention relates to tenter apparatus hereinafter stenter apparatus for stretching a web to produce a relatively thin film, and especially to stenter apparatus comprising laterally spaced pairs of endless chains or bands each of which includes film clip holders running on rails, facing inner or feed runs of the chains or bands defining a film conveying passage comprising in succession an inlet section, a diverging film stretching section and an outlet or relaxation section, end sprockets being present at the ends of the inner feed run and the outer or return run of the chains. Such apparatus is hereinafter referred to as stenter apparatus of the type aforesaid.
Stenter apparatus of the type aforesaid in shown in UK Patent Specification No. 1015499 wherein the film holder rails are mounted on a supporting frame structure. For heating and/or cooling of the film, the stenter of No. 1015499 includes chamber means through which passes film gripped on the feed runs of the chains, the chamber means consequently housing the portions of the feed runs of the chains defining at least parts of the film feed, stretching and outlet sections. For operational and economy reasons the return runs of the chains are located outside the chamber means. Additionally No. 1015499 includes adjustment means for lateral movement of the feed and return runs of each chain in parallelogram manner so that the spacing between the opposed feed runs can be varied so as to enable for example the degree of stretch imparted by the apparatus to be varied. More specifically the rails comprise a series of pivotally jointed sections with a carrying block at each joint, and the adjustment means includes transverse control shafts threaded to said carrying blocks and rotatable to move the blocks for parallelogram motion f the feed and return rail sections of each endless chain, the adjustment means being such that opposed lateral movements of the two chains are effected simultaneously relative to the centre-line of the stenter. Equivalent lateral movements are imparted to the end sprockets. The necessary apertures at the ends of the chamber means for the feed runs are of sealed slot form to accommodate the lateral movements of the feed runs. The stenter apparatus of No. 1015499 has the following disadvantages, namely:
1. The stenter apparatus is complex, in particular requiring a considerable number of pivotal joints for the rail sections to permit desired width variations and patterns to be obtained along the length of the film conveying passage; and
2. The end sprockets have to be of relatively large diameter to allow a reasonable degree of lateral movement of the inner feed run if the outer return run which moves laterally with the feed run is not to foul the chamber means.